The focus of this section is on the discovery and functional analysis of the network of genes involved in the development and operation of the nervous system. The differentiation of progenitor cells into a range of neurotransmitter-expressing phenotypes requires distinct sets of transcription factors, and we have cloned two such putative factors. One is a member of the kruppel class of the zinc finger superfamily named rKr2. rKr2 is localized in the nuclei of a discrete population of neurons, granule cells. In mature oligodendrocytes, rKr2 is also expressed but is restricted to the cytoplasm. The function of this zinc finger protein is being investigated by analyzing transgenic mice expressing a dominant negative version of rKr2, screening for interacting proteins in a yeast two-hybrid system, and identifying the set of target genes that this zinc finger protein recognizes. We have also cloned a DNA binding protein that recognizes a cis regulatory element of the enkephalin gene using a novel application of the phage display methodology. Cell culture systems are an invaluable tool in neural cell biology, but the difficulties in obtaining purified populations of neural cells have limited some studies. We have developed a transgenic mouse strategy for isolating pure populations of oligodendrocytes and Schwann cells. Several transgenic mouse lines were created in which a myelin gene promoter drives expression of a GEO reporter, thus enabling an antibiotic selection for oligodendrocytes or Schwann cells. The surviving glial cells can also be distinguished from other cell types by their expression of the lacZ gene. Employing this transgenic model, we have cultured a source of oligodendrocyte progenitor cells from the rhombencephalon region of embryonic mouse brain at E12.5. Ongoing studies with the GEO transgenic mice will pinpoint the origins of the Schwann cell lineage in the developing peripheral nervous system and the oligodendrocyte lineage in the brain.